1. Field of the Invention
The present invention relates to a position measuring device, in particular for employment in connection with a lithographic system. The present invention moreover relates to a method for position determination in a position measuring device.
2. Description of the Related Art
It must be possible by a position measuring device in a lithographic system to determine the position of a movable object in such a way that the time of the measurement, as well as the position of the movable object are very precisely fixed. A lithographic system operating in accordance with the scanner principle moves a wafer table and a photo-masking table, for example, in relation to each other, and in the process exposes only a portion of the area to the photo-mask to be represented on a wafer. Spatial or chronological errors in positioning, and therefore in position determination, result in overlay errors of the structures to be represented with respect to structures already present on the wafer.
In such a lithographic system a central control requests a positional value from a position measuring device at fixed times (for example every 50 μs), which then must output the position of the movable object (for example the mask table) at the time of the position value request as the response. Because of the high displacement speeds customary today (approximately 2 m/s for the mask table), an inaccuracy of one nanosecond in the chronological determination of the positional value then means an error of two nanometers in the position determination, which approximately corresponds to the demands made on such a position measuring device.